1. Field of the Invention
This invention relates in general to ion beam etching and deposition of thin film, and more particularly to a method and apparatus for in-situ monitoring of ion energy distribution for endpoint detection via capacitance measurement.
2. Description of Related Art
The processing of semiconductor wafers to create integrated circuits and devices such as sliders and disk overcoats involves a sequence of processing steps which build up or remove layer structures. These processing steps include the deposition of metals, dielectrics and semiconductor films, the creation of masks by lithography techniques, the doping of semiconductor layers by ion implantation and the etching of layers for selective or blanket material removal.
The semiconductor manufacturing industry continues to increase the functionality and performance of integrated circuits by increasing the number of circuit elements within each integrated circuits chip. Further, the size of devices structures continue to decrease. As the size of the elements decrease and their density increases, the processing steps become more critical. For example, wet etching techniques are generally inadequate for feature sizes less than 2 microns because wet etching is an isotropic process which etches equally well in all directions. With smaller feature sizes, the isotropic etching causes undesirable undercutting of the masking layer and possible destruction of closely spaced elements.
To address the need for a more anisotropic processes, ion beam and reactive ion etching and deposition are increasingly being used. These techniques involve the application of RF power to a cathode and typically allow the anode to electrically float. In either case, however, positive ions formed in the plasma are accelerated towards a wafer by a self-biased negative cathode to provide an effective anisotropic etch or deposition to the wafer.
Predicting or measuring when a desired layer of an element has been etched or deposited to the desired level is an important aspect of such ion beam and reactive ion etching processes. For example, it is desirable to measure when a layer of an element has been etched to a proper depth, i.e., when the etch process has reached an "end-point". Prediction or measurement of the end-point is needed to prevent damage to a wafer caused by excessive over-etching or to provide the proper etch depth. End-point detection is particularly crucial in ion beam and reactive ion etching, because this detection tends to have much lower selectivity than the comparable wet etching processes.
Prior end-point detection methods have typically monitored the emission spectra of the plasma, the surface layer of the wafer, or one of the operating parameters of the plasma system itself. For example, to form a reactive ion or ion beam, the equipment usually initiates and maintains a plasma in a gas mixture containing either fluoro-hydrocarbons or inert gas. The system maintains the plasma at a positive potential with respect to ground. Positive ions are then electrostatically extracted from the plasma to form a beam of energetic ions. Control of ion energy requires adjustment of the plasma potential, i.e., bias voltage, which is known as the beam (plasma) voltage. The beam of ions is then aimed at the substrate (normally remote from the plasma). Deposition and etch rates vary in proportion to the ion-beam (plasma) current density. With the gridded ion source, the gas flow, discharge voltage, beam voltage, beam current, accelerator voltage, and neutralization current can all be independently controlled over wide operating ranges. The specific type of gas used, its rate of flow into the ion source, and the magnitude of the discharge voltage will determine the ion species formed in the plasma. The beam voltage controls ion-impact energy at the substrate, for a given beam shape and beam/substrate geometry, the beam current controls the deposition and etch rate. The accelerator voltage can be used to adjust the beam shape, i.e., focus the beam, or adjust the beam's current density at the substrate.
Nevertheless, the stability of various plasma processes can be problematic. The intensity of an RF plasma can be difficult to control, and fluctuations lead to increased process tolerances or to decreased yields. The most important indicators of plasma deposition or etch rates are the flux of ions that strike the material surface, and the energy distribution of those ions. Optical and electronic techniques for monitoring plasma strengths are often utilized. However, these techniques are at best indirect methods and therefore do not provide adequate monitoring and measurement capabilities. The measurement of ion energy can be used to determine endpoint and uniformity of etch and deposition processes that use plasma.
It can be seen then that there is a need for a method and apparatus that provides in-situ monitoring of both the ion flux and the ion energy distribution of plasma processes.